Valve actuators are known of the kind in which an hydraulically or pneumatically activated piston is used to drive a shaft in either one direction or the other through an operative stroke of, say, 90.degree., via a rack and pinion mechanism. It is usual to employ a double-ended arrangement using two racks working on opposite sides of a common pinion so as to provide a balanced driving couple. Such devices are made in a variety of sizes and are expensive to produce because of the high grade components needed to deal with the hydraulic or pneumatic pressures and torques involved. It is an object of this invention to provide an actuator which can be produced at less cost by employing a system of construction using fewer parts and parts of a nature lending themselves to ease of manufacture by repetition techniques and a simplified system of assembly. By "repetition techniques" in this context are meant plastic moulding, die casting, pressing and the like processes.
Such actuators may be of the so-called double acting kind, in which hydraulic or pneumatic pistons are used to drive the mechanism in both directions or of the kind sometimes called "fail safe" in which the mechanism is driven in one direction hydraulically or pneumatically but is driven in the other direction by spring return means. This invention is applicable to both such kinds as will appear hereinafter.